The header traditionally contains: 1. a pointer to the class 2. the object's lock 3. some GC bits 4. the identity hash code In a naive implementation, the

The header traditionally contains: 1. a pointer to the class 2. the object's lock 3. some GC bits 4. the identity hash code In a naive implementation, the class pointer is a machine word, the lock is multiple machine words, the GC bits are a few bits, which get rounded up to a word for alignment, and the hash is 32 bits. Object headers used to be even bigger than 128 bits! There have been improvements: compress the class pointer, use a lazily-inflated lock which takes up only one word in the header, make the identity hash smaller, and make the lock and the identity hash share space (when the lock is deflated, the identity hash lives in the shared slot; when the lock is inflated, the slot has a pointer to a lock structure which also contains the identity hash). The change in this JEP is effectively to extend the inflation-dependent space sharing scheme to include the class pointer too. When the lock is deflated, the header contains the class pointer and the identity hash; when it is inflated, it is a pointer to the lock structure, which contains the class pointer and identity hash. I suppose this means that virtual method calls through a locked object will involve an additional indirection - but this should be rather easy for a compiler to optimise, because it can safely hoist the loading of the class pointer.